        part_var = &
              gx(-2) * grid_var(cell_x1-2) &
            + gx(-1) * grid_var(cell_x1-1) &
            + gx( 0) * grid_var(cell_x1  ) &
            + gx( 1) * grid_var(cell_x1+1) &
            + gx( 2) * grid_var(cell_x1+2)
